I did this about a year ago to link up two 4 ohms cabs (Messa 1516BE & Eden D215XLT) to an Ashdown ABM500.
Even though this changes the total impedance to 8 ohms the results were very good with no appreciable loss of volume, in fact the volume of speaker's moving air more than made up for the reduced power.
Unfortunately the link I used was on Bass Players old site and it no longer works.
Did a search on the new site, but it came up with nothing.

The cable is easy to make up if youre fairly compliant with a soldering iron. Somewhere at home I have a print out of the diagram from Bass Player, so if no one else can give you any "off the top their head" help, let me know by PM or email and I will try to dig it out, scan it and email it to you.

Interesting, but I tried a couple of local amp tech engineers from a couple of shops and they didn't even know this could be done, and advised me to either buy new cabs or change all the speakers.